It is required on an aircraft to have reverse pressure differential relief valves to automatically prevent a negative pressure differential that would damage the structure. It is normal to use a skin-mounted valve that opens inwardly. The valves are sized for an unpressurized condition, no inflow, and maximum aircraft descent. The valves require a relatively large geometric area, due to a low flow coefficient through the valve, and as the weight of the valve installation is proportional to the size of the pressure shell cut-out, it makes for a relatively heavy valve. The saving of weight is a continuing consideration in the design of aircraft. With the advent of the wide body aircraft, the pressurized volume has increased, negative pressure relief area requirements of valves has increased, and this calls for a heavier valve for negative pressure relief. It has been found that a cooperating double poppet valve will provide a negative pressure relief valve for aircraft that will effect considerable weight savings.
U.S. Pat. No. 2,748,855 to Siems et al provides for blow-out safe aircraft cargo doors that prevent accidental release of the doors in flight. The doors have relief valves that must be opened to equalize pressure on both sides before the door can be opened.
U.S. Pat. No. 2,815,705 to Jensen calls for a butterfly type valve on an aircraft that provides pressure relief, vacuum relief, and also acts as a dump valve.